In recent years, there has been a demand for batteries and battery packs with high capacity, high voltage and high output and high safety as electronic devices have been diversified. As means for providing particularly safe batteries and battery packs, a battery is generally provided with a PTC element or a temperature fuse for preventing an increase of a battery temperature and protection means for cutting off a current by sensing an internal pressure in the battery. Further, a safety circuit is installed in a battery pack to control the battery so as not to become defective.
However, even if the conventional protection means is provided, abnormality may occur in the battery and high-temperature and pressure gas may be blown from the inside of the battery. In such a case, it is thought that a casing of a pack containing the battery is broken, melted or burnt, whereby the high-temperature and pressure gas leaks to the outside of the battery pack or is burnt and spreads burning inside and outside the battery pack to increase damage.
As a method for preventing such a phenomenon, there has been proposed a method by which high-temperature pressure gas released from the battery is released to the outside of the casing after reducing the temperature and pressure of the gas and diffusing the gas in a casing in a battery module containing a plurality of batteries in the casing (patent literature 1) or a method by which a plurality of electric cells each having a safety valve for releasing gas when pressure in the cell increases to or above a specified value are connected into an electric cell group, a duct for exhausting the gas released from the electric cells to the outside is provided with an inflatable bag, pressure of the exhausted gas is decreased by the inflation of the bag in the case of generating a large quantity of gas, thereby releasing the gas to the outside without damaging the duct (patent literature 2).
However, since the gas released from the batteries has high temperature, high pressure and high inflammability, it may ignite upon being brought into contact with or mixed with oxygen in the air in some cases. Accordingly, in the case of using the means for reducing the temperature and pressure of the exhaust gas in a space as disclosed in patent literatures 1, 2, the gas is mixed with oxygen in the casing containing a plurality of batteries or in the exhaust duct, whereby the ignited gas comes to have higher temperature and pressure. Therefore, even other batteries start a runaway effect and the entire battery pack is broken, thereby increasing damage.
Patent Literature 1:
    Japanese Unexamined Patent Publication No. 2005-322434Patent Literature 2:    Japanese Unexamined Patent Publication No. 2005-339932